Feedback amplifiers have long been used in a variety of electronic circuits. By means of a feedback element, circuit stability and linearity can be improved. To accomplish this end, it is well known to use negative feedback in combination with resistive feedback elements.
It has also been recognized that lossless feedback elements, such as transformers, can be used to improved amplifier performance without the attendant losses associated with resistive elements. Representative examples of such circuits can be found in Sontheimeyer et al., U.S. Pat. No. 3,426,298; Norton, U.S. Pat. No., 3,624,536; or Norton et al., U.S. Pat. No. 3,891,934.
Another example of such an amplifier can be found in Ezzeddine et al., published April 1990 in the IEEE Transactions on Microwave Theory and Techniques, entitled "NMMAC C-Band FET Feedback Power Amplifier". The amplifier of this article is intended to be used in a frequency range on the order of 3-5 gigahertz, in connection with satellite communication.
It is also been known to combine an input coupling element with an output coupling element and a gain element to provide a low loss feedback mechanism. FIG. 1 illustrates a suggested prior lossless feedback approach which confines an input coupling element Cl, an output coupling element C2, and a gain element G.
The system of FIG. 1 could be expected to have a loss of 10 dB associated with each coupler. As a result, close loop gains less than 20 dB are particularly difficult to achieve.
Thus, there continues to be a need for amplifiers operable at microwave frequencies, which can provide relatively low closed loop gains on the order of 10 dB or so. Such amplifiers, if achievable, will provide improved linearity and enhanced intermodulation performance.